For many years, metal roofing systems, specifically stainless steel and low carbon steel sheet, in various sheet gauge thicknesses, have been used for a variety of roofing applications. Stainless steel and low carbon steel are typically treated with a terne metal alloy to reduce the corrosion rate of the roofing system. When the terne coated steel sheets are assembled into a roof covering, adjacent sheet edges are folded over one another and the seam then formed, typically a standing seam, usually soldered vis-a-vis the terne coating to produce a waterproof joint. Today, the terne coated steel sheets are either preformed or formed at the job site onto roofing pans with bent edges which abut edges of adjacent pans which are then pressed or rolled into a watertight seam. Similarly, caps, cleats, etc. are likewise formed from the terne coated sheet. In addition to providing for soldering of the seams, the terne coating inhibits rusting or oxidation of the metal sheet which would otherwise occur over time.
Terne or terne alloy is a term commonly used to describe an alloy containing about 80% lead and the remainder tin. The terne alloy is conventionally applied to the metals by a hot dip process wherein the metal is immersed into a molten bath of terne metal. The terne coating greatly inhibits the formation of ferrous oxide on the metal thus preventing corrosion and extending the life of the metal. The corrosion resistive properties of the terne alloy result from the stability of elemental lead and tin and the lead-tin oxide which forms from atmospheric exposure.
Although terne coated stainless steel or low carbon steel have excellent corrosive resistive properties and have been used in various applications such as roofing, the terne coated roofing materials have recently been questioned due to environmental concerns. The terne coating contains a very high percentage of lead and commonly includes over 80 weight percent of the terne alloy. Although the lead in terne alloys is stabilized, there is concern about leaching of the lead from the terne alloy. As a result, terne coated materials have been limited from use in various applications, such as aquifer roofing systems. The concern of lead possibly leaching from terne coated roofing systems renders normal terne coating inadequate and undesirable as a metal roofing coating for these types of roofing applications.
Roofing systems made of uncoated sheet metal or stainless steel have been used for aquifer roofing systems. However, unprotected sheet metal or stainless steel tend to oxidize after being exposed for a relatively short time to the atmosphere. The oxidation or corrosion of the sheet metal or stainless steel may combine with the runoff water and adversely affect the taste and color of the water. Other metal such as copper can be used as the roofing material; however, copper, although more corrosion resistant than low carbon steel, will eventually combine with oxygen and carbon dioxide in the atmosphere to form a protective green or bluish-green layer. Although the protective layer greatly reduces the further oxidation of the copper, a small amount of the protective layer may cosine with the runoff water and affect the color and taste of the water. Other metals which resist oxidation such as titanium, chromium, gold or nickel are too expensive and/or unsafe to use for roofing materials.
Another disadvantage of terne coated materials is the softness of the terne layer. As noted, terne coated metal sheets are commonly formed into varying shapes. The forming machines used to bend the metal sheets periodically damage the terne coating during the bending process, since the terne coating is susceptible to damage due to the abrasive nature of the forming machines.
Another disadvantage of terne coated low carbon steel or stainless steel is the difficulty of forming the roofing materials into intricate designs. Each roof of a building has a variety of intersecting slopes and configurations. In addition, many roofing systems include simple and/or complex ornamental designs. Because of the different designs of the roofing systems, much of the roofing material is formed on sight. Stainless steel and low carbon steel are strong and rigid materials which cannot be easily formed on sight into complex shapes. As a result, complex and ornamental shapes must be preformed prior to installation of the roof system.
A further disadvantage of using normal terne coated materials is that newly applied terne is highly reflective to light. Use of terne roofing materials on buildings near or within an airport can produce a certain amount of glare to pilots taking-off and landing. Due to the highly stable nature of terne alloys, terne coated metals take about one and one-half to two years before oxidation of the terne begins to dull the terne alloy surface. The present invention deals with these disadvantages of normal terne coated roofing sheet material.